Magnetostriction is a property of ferromagnetic materials that causes them to change shape when subjected to a magnetic field. Magnetostrictive materials can convert magnetic energy into kinetic energy, or the reverse, and are used to build various actuators and sensors.
For active magnetostrictive testing, elastic waves are launched and reflected echoes of the waves from defects such as corrosion or cracks are detected. Various magnetostrictive actuators have been designed to generate guided waves for testing various types of structures. The guided waves may be longitudinal, torsional, or shear, and the type of wave may depend on whether the structures are rods and cables, pipes, or plates.
Because many systems use the same device for actuating the guided waves as for receiving the reflected waves, magnetostrictive actuator/sensors are often referred to as simply “sensors”. Combined actuator/sensor devices are also often sometimes referred to as “probes”.
Ferromagnetic coupling of a magnetostrictive sensor to the material being tested is an important operative feature of magnetostrictive test systems. Some sensors may make use of magnetostrictive properties of the material being tested, and do not require a magnetostrictive coupling interface.
However, many sensors are made more effective, or are adapted for testing non-ferromagnetic metals, by mechanical coupling of a ferromagnetic material to the material being tested at areas where the sensors are to be placed. This may be achieved, for example, by coating the surface of the material to be tested with a coat of ferromagnetic material or by bonding a ferromagnetic medium to the surface of the material. Some sensors incorporate a ferromagnetic material into the sensor itself.